This invention relates to liquid crystalline polymer composites and in particular to liquid crystalline polymer composites useful as dielectric substrates in circuit materials, circuits, and multi-layer circuits.
As used herein, a circuit material is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, bond plies, conductive layers, resin coated conductive layers, and cover films. Circuit laminates, bond plies, resin coated conductive layers, and cover films comprise dielectric materials formed from a thermosetting or thermoplastic polymer. The dielectric material in a bond ply, resin covered conductive layer, or cover film may comprise a substantially non-flowable dielectric material, i.e., one that soften or flows during manufacture but not use of the circuit The dielectric material in a circuit laminate (i.e., a dielectric substrate), in contrast is designed to not soften or flow during manufacture or use of the circuit or multi-layer circuit. Dielectric substrate materials are further typically divided into two classes, flexible and rigid. Flexible dielectric substrate materials generally tend to be thinner and more bendable than the so-called rigid dielectric materials, which typically comprise a fibrous web or other forms of reinforcement, such as short or long fibers or fillers.
Circuit laminates further have a conductive layer fixedly attached to a dielectric substrate layer. When a second conductive layer is disposed on the other side of the dielectric layer, the material is often referred to as a double clad circuit laminate. Patterning a conductive layer of a circuit laminate, for example by etching, provides a circuit, or in the case of a double clad circuit laminate, a double clad circuit. One or both of the conductive layers of a double clad laminate may be processed to provide circuit layers.
The aforementioned circuit materials and circuits may be combined in various configurations to provide multilayer circuits. “Multilayer circuits” as used herein refers to materials having at least two dielectric layers and at least two conductive layers, wherein at least one of the conductive layers is circuitized, and is inclusive of both subassemblies used to form finished circuits and the finished circuits themselves.
In one simple form, a multilayer circuit includes a double clad circuit and a resin coated conductive layer, wherein the dielectric material of the resin coated conductive layer is disposed on a circuit layer of the double clad circuit. In another simple form, a multilayer circuit includes a first circuit and a second circuit joined by a bond ply disposed between the circuit layer of the first circuit and the dielectric substrate of the second circuit. Typically, such multilayer circuits are formed by laminating the circuit(s) and/or circuit material(s) in proper alignment using heat and/or pressure. Bond plies can be used to provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In place of a conductive layer bonded to a circuit with a bond ply, the multilayer circuit may include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multilayer structures, after lamination, known hole forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
Dielectric materials are typically divided into two classes, flexible and rigid. Flexible dielectric materials generally tend to be thinner and more bendable than the so-called rigid dielectric materials, which typically comprise a fibrous web or other forms of reinforcement. While there are a variety of rigid circuit materials available today, for example, FR4 epoxy glass laminates, and the like, there is a growing demand for circuit materials for high performance (high frequency) applications, that is, applications operating at 1 gigahertz (GHz) or higher. High performance applications require, among other things, circuit materials having low dielectric constants for low propagation delay, lower cross talk and higher clock rates, low dissipation factor (Df) for low attenuation, better signal integrity, and lower power consumption in portables. In addition, the circuit should be non-flammable, preferably achieving a rating of V-1 or better in the Underwriter's Laboratory UL-94 flammability test. In addition, the circuit materials preferably have low coefficients of thermal expansion (CTE) in all directions to provide good dimensional stability and enhanced reliability, e.g., plated through-hole reliability. The circuit materials further preferably have excellent copper peel strength, particularly at high temperature.
While there are now many materials, such as the RO4350 made by Rogers Corporation, which satisfy these properties, they all contain halogenated additives to achieve the desired level of flame retardancy. As use of halogenated flame retardants is being phased out for environmental reasons, it has become apparent that new rigid dielectric materials are needed for use in circuits, and in particular new rigid dielectric materials that have acceptable high performance at high frequencies as described above and are flame retardant without the use of halogenated additives.